Running out of sugar

Training, November 18, 2017

We had a very interesting thing happen in one of our Saturday classes. Two of our riders, J and D we’ll call them, experienced running out of energy and getting very hungry during our two hour training session. This isn't a super hard workout so what happened to make them feel hungry? What does their experience mean for their group riding?

We had a very interesting thing happen in one of our Saturday classes. Two of our riders, J and D we’ll call them, experienced running out of energy and getting very hungry during our two hour training session. Now this isn’t a super hard workout, Saturday is typically 70% of MAP which is the top of endurance, but it is a challenge to the aerobic system. As we have written in previous articles not everyone is able to stay at an endurance heart rate when doing endurance wattage. What does their experience mean for their group riding? What happened to make them feel hungry?

Let’s look at the workout. There was a nice steady warm-up and then the bulk of the work was an hour long block. During this block the riders would start with a 5-minute effort, at the end of which they stop and recover to 70% of their max heart rate. If recovery is fast, say 30sec as it was for J and D, then they will progress to doing a 10-minute effort. At the end of 10-minutes they stop and check how quickly they recover to 70% again, and if it is fast start adding 2-minutes to their block of work. In this way the time you spend working increases, but with very little rest between the blocks of work.

J and D are two strong riders and so can push a decent amount of watts at 70% of their MAP. They were pushing upwards of 215-230w, so top of endurance bottom of tempo for many riders. Their heart rates though were pushing up towards 85% of their max, and even a bit above. We know that when we start to flirt with this threshold heart rate, threshold is 85-90% of our max heart rate, that the fuel we are burning to do the work starts to move towards predominantly sugar. At tempo and below we burn mainly fat and some sugar, but as the work gets harder and we cross into a threshold effort we burn more sugar to do the work.

So what happened in the workout for J and D?

As their heart rates rose across the hour they burned more and more sugar. And with the way the workout is structured there is very little rest to recover. Over time they burned through their glycogen stores - the sugar - in their muscles. What is important to understand is that the body can store only about 2000 calories of sugar, but has lots of fat on hand to burn. More fat than we can burn on a single ride. And that while riding we can only restore a limited amount of sugar back into the system.

Why is this important to understand?

Think about this in the context of your local weekend group ride. The more fat you can burn on a ride the more sustainable it will be. You will be less likely to run out of energy. This ability to burn fat is why the strength of your tempo ability is so important. The stronger your tempo ability - the more watts you can push at a tempo heart rate - the more fat you can burn on your rides. The more fat you burn the more sustainable the ride, the more sugar you have left when it really matters and need to punch it on a hill.

Now think about where your heart rate is for a lot of your group rides. Probably above tempo, with a good deal of time spent at threshold. This might be why you always find yourself starting out feeling good but then struggling later on. Even if you have a good fuelling strategy, taking in the maximum number of calories you can each hour, you cannot replenish what you are using.

So, if you spend all your time training on your group rides at threshold and above, as many athletes do, you are training a predominantly sugar burning energy system. By training this way you will never improve your fat burning capabilities. You can only develop your fat burning by riding at a proper endurance or tempo heart rate. This might mean doing some rides without the group!